1. Field of the Invention
The present invention relates to a Ni based forged alloy, and a turbine disc, a turbine spacer and a gas turbine each using this alloy, and particularly to a Ni based forged alloy which is excellent in a high-temperature fatigue property, has a moderate macrosegregation property, and enables to control the crystal grains of the alloy and to manufacture a large-sized member of the alloy.
2. Description of Related Art
With a rise in an efficiency of gas turbines, Ni based heat-resistant alloy which has an excellent high-temperature strength has been used for their various members. Ni based heat-resistant alloy contains solute strengthening elements such as W, Mo and Co in a large proportion, and precipitation strengthening elements such as Al, Ti and Nb in a large proportion. These elements contribute intensely to the strength of the alloy. In particular, a γ′ phase made of Ni3Al, and a γ″ phase made of Ni3Nb which are precipitation strengthening phases can be precipitated minutely into innumerable regions of a parent phase of the alloy. The precipitation is very effective for improving the alloy in the high-temperature strength. The γ′ and γ″ phases are stabilized by Al, Ti and Nb. In the design of the high-temperature strength of the Ni based alloy in developments of the alloy, a main attention is paid to a phase stability of these precipitation strengthening phases.
However, as each of these solute strengthening elements and precipitation strengthening elements is added to a material for Ni based alloy in a larger proportion, segregation is more easily caused while the material is solidified. Thus, a large-sized member is less easily produced therefrom. Therefore, the use of the high-strength Ni based alloy is restricted mainly into use for small-sized members such as members for aircrafts, and a rotor blade and a stator blade for a land. For example, alloy 718 has widely been put into practical use as a Ni based forged alloy species having the excellent high-temperature strength by effect of its γ′ and γ″ phases. However, because of Nb and Mo added thereto, the alloy has a degrading macrosegregation. Thus, when the alloy is used for a relatively large-sized member, it is necessary to use an alloy-producing method involving, for example, the control of the alloy-material-solidifying speed. Furthermore, in the production of a large-sized Ni based alloy primary product having a weight of 5 tons or more, many Ni based alloy species cannot be used since restriction is imposed onto conditions for the solidification in order to continue an operation for the production stably.
According to Japanese Patent Application Laid-Open No. 2012-117122 (Patent Document 1), alloy 718 is improved in macrosegregation property. A cause that the macrosegregation is caused in the alloy would be that a solute element is distributed in a solid-liquid interface of the alloy to cause a change in a density deviation of a melt of the alloy. As any element having a larger atomic weight than the average atomic weight of entire elements in the melt alloy is decreased in the added proportion of the element, the melt density deviation in the alloy tends to be reduced to restrain the segregation. Contrarily, as any element having a smaller atomic weight is increased in the added proportion of the element, the melt density deviation tends to be reduced to restrain the segregation. Thus, according to Patent Document 1, the alloy can attain compatibility between an excellent high-temperature strength and a large ingot manufacturability by balancing its elements different from each other in macrosegregation tendency (Al, Ti, Nb and Mo) with each other, thereby making the melt density deviation close to zero to restrain these elements from segregating.
Another method for improving such an alloy in macrosegregation property is a method of controlling the respective distribution coefficients of its individual elements, as disclosed in Japanese Patent Application Laid-Open No. 2009-191301 (Patent Document 2). An element having a larger or smaller atomic weight than the average atomic weight of entire elements in the alloy melt promotes generation of the macrosegregation further as the distribution coefficient of the element is apart farther from the value “one”. However, it has been found out that the distribution coefficient of a specific element is controllable by changing the proportion of an element other than the specific element added to the alloy. According to Patent Document 2, the addition of Co makes the respective distribution coefficients close to the value “one”, the respective distribution coefficients being of not only Al, Ti and Nb but also W which promotes the generation of the macrosegregation largely.